Bromobenzene-3,4-oxide is sufficiently stable to escape the liver and reach systemic blood. We have identified bromobenzene-3,4-oxide in blood by trapping it as its two glutathione conjugates in the presence of purified glutathione S-transferases and 35S-glutathione. As much as 2.55 nmoles of the epoxide per ml of blood withdrawn from the retro-orbital vein was found at 2-3 hours after bromobenzene administration to male Sprague-Dawley rats (weight 150-200 g.)